1. Field of the Technology
The present disclosure relates to a chemical method for removing defects from the surface of glass coated electrical steel.
2. Description of the Background of the Technology
Electrical steel is an iron alloy which may have from zero to 6.5 percent by weight of silicon. Commercial alloys usually have silicon content up to 3.2 percent by weight, as higher concentrations of silicon may exhibit brittleness during cold rolling. Manganese and aluminum can be added up to 0.5%. Increasing the amount of silicon inhibits eddy currents and narrows the hysteresis loop of the material, thus lowering the core losses. However, the grain structure hardens and embrittles the metal, which adversely affects the workability of the material, especially when rolling it. When alloying, the concentration levels of carbon, sulfur, oxygen and nitrogen must be kept low, as these elements result in the formation of carbide, sulfide, oxide and nitride particles. These particles, even in particles as small as one micrometer in diameter, increase hysteresis losses while also decreasing magnetic permeability. The presence of carbon has a more detrimental effect than sulfur or oxygen. Carbon also causes magnetic aging when it slowly leaves the solid solution and precipitates as carbides, thus resulting in an increase in power loss over time. For these reasons, the carbon level is kept to 0.005 percent by weight or lower. The carbon level can be reduced by annealing the steel in a decarburizing atmosphere, such as hydrogen.
Electrical steel is available as grain oriented electrical steel (GOES) and non-oriented electrical steel. GOES is used for transformer cores and in certain other electrical applications. GOES sheet is processed so that the crystal grain orientation of the sheet is tightly controlled and the sheet properties are optimized in the rolling direction. As a result of the grain orientation, the magnetic flux density in GOES sheet may be increased by about 30 percent in the coil rolling direction, although the magnetic saturation may be decreased by about 5 percent. GOES sheet is usually manufactured in the form of cold-rolled strips less than 0.35 mm thick. The strips are stacked together as “laminations” to form a core. The assembled cores may be used as laminated cores in electrical transformers.
In the conventional process of manufacturing transformer cores from GOES sheet, a glass film of silicon-rich oxide is provided on surfaces of the sheet. After final cold rolling, the GOES sheet undergoes a final normalizing in an atmosphere of hydrogen and nitrogen. This forms a thin oxide layer on the surface that contains silicon and iron. The GOES sheet surface is then coated with magnesium oxide (MgO) powder. During the final annealing in a hydrogen atmosphere, silicon in the thin oxide formed in the normalizing step reacts with the MgO and forms a thin, uniform, silicon-rich insulating layer of crystalline forsterite (Mg2SiO4) on the sheet surface. The forsterite coated GOES sheet is scrubbed cleaned. GOES sheet with a forsterite layer is generally known in the art as “mill glass” or, more simply, “scrub material”.
For use in transformer cores, the scrub material is top coated with an additional electrically insulating layer. Monomagnesium phosphate (Mg(H2PO4)2), with or without inorganic filler materials, is one example of an electrically insulating coating that is applied over a mill glass coating. The top coat is added to increase the electrical resistivity of the surface and improve the electrical properties of the sheet.
Defects in the electrically insulating coatings on GOES sheet can allow current to leak through the coatings. These electrical shorts are problematic if, for example, the sheet is intended for transformer applications. Defects in the electrically insulating coatings can act as short circuit paths for current to flow between sheet laminations in an electrical transformer core, reducing electrical efficiency and increasing the generation of waste heat.
One type of mill glass coated GOES sheet defect is an “iron mound” defect. Iron mounds form during the steps of producing mill glass on electrical steel sheet. It is believed that iron mounds originate from iron-rich oxides produced during annealing of GOES. The iron-rich oxides are reduced in the dry hydrogen environment of the tunnel furnace high temperature soaking cycle. The resulting electrically conductive defect is rich in metallic iron and may protrude through the mill glass coating. A scrub material surface including iron mounds appears gray in color, with numerous small bright spots. FIGS. 1A-1C show the general morphology of iron mound defects on scrub material when viewed through a light optical microscope. The defects are raised from the surface and appear to comprise metallic iron and entrained iron oxides. As best shown in FIG. 1A, a gouge or tail in the base glass is often associated with an iron mound, running transverse to the rolling direction. The gouge is secondary to the iron mound and likely formed while the steel was tightly coiled, arising through friction between laminations during coil handling, and not during line processing which typically produces damage along the rolling direction. Iron mounds typically are 50-200 microns in diameter, are generally round or elliptical, and may protrude from the sheet surface by approximately 50 microns. 50 microns is considerably thicker than the entire electrically insulating glass coating provided on a finished GOES sheet, as the mill glass and the top coating are both only a few microns thick.
FIG. 2A is a secondary electron scanning electron micrograph of an iron mound on a GOES sheet, and FIG. 2B is a back scattered electron micrograph of the same iron mound. Heavier, i.e., high atomic number, elements backscatter electrons more strongly than light, i.e., low atomic number, elements and thus appear brighter in the image of FIG. 2B. Therefore, it can be inferred that in FIG. 2B the brighter portion of the image represents an iron mound, and the darker portion of the image represents the mill glass coating. FIG. 2C is a cross-section through an iron mound defect. The layer in the upper portion of FIG. 2C is a coating used to prepare the cross-section and does not represent the mill glass. The mill glass coating is not evident in FIG. 2C, as it is too thin to distinguish.
SEM microanalyses of an iron mound are shown in the scanning electron micrograph of FIG. 3A and the energy dispersive SEM X-ray maps of FIGS. 3B-3G. The X-ray maps of FIGS. 3B-3G are maps for the elements Mg, Si, O, Mn, Fe, and S, respectively. For each elemental map, the brightness of the image is determined by the presence and concentration of the element scanned for in the image. Examination of FIGS. 3B-3G indicates that the iron mound defect is composed mostly of iron (FIG. 3F). The presence of a substantial amount of oxygen is not detected in the iron mound (FIG. 3D), suggesting that the iron mound defects are comprised of electrically conductive iron metal. Regions outside of the iron mound defect are high in magnesium and silicon (FIGS. 3B and 3C), indicating the presence of the forsterite mill glass coating. Sulfur-rich particles are visible in the mill glass around the iron mound, and there also are indications in the images of sulfur in particles embedded in the mounds. The large sulfur concentration observed indicates that sulfur in the iron mound is probably a remnant from additives present in the MgO coating from which the forsterite mill glass forms.
Iron mound defects are very difficult to cover with one application of a phosphate top coating. The locally high electrical conductivity of the coated surface resulting from iron mounds generally dictates rework in the form of a second application of phosphate coating. Applying two layers of the top coat increases costs and production lead time and decreases the stacking factor of a GOES sheet product used in a transformer core, for example. As such, it would be advantageous to avoid the need to apply an additional top coat.
“Free iron” particles can form on stainless steel surfaces that have contacted ferrous tooling during processing. Ferrous tooling can embed the free iron particles into the stainless steel surface. The free iron particles can rust, which can lead to corrosion of the underlying stainless steel. Once corrosion of the underlying stainless steel begins, the corrosion can continue without the presence of the free iron. Free iron can be removed from stainless steel surfaces using a conventional passivation technique. ASTM 967-05 defines passivation as the chemical treatment of a stainless steel with a mild oxidant, such as nitric acid solution, for the purpose of removing free iron or other foreign matter from the surface, but which is generally not effective in removal of heat tint or oxide scale from the surface. The oxidizing nature of the acid encourages the formation of a native chromium oxide film, or passivation layer, which is responsible for the corrosion resistance of stainless steel. Although passivation is effective in removing iron deposits from a stainless steel surface, the technique is ineffective at removing iron from GOES and non-oriented electrical steel due to the absence of chromium in the steel.
U.S. Pat. No. 4,123,337 (“the '337 patent”) discloses an electrolytic process that may be applied to GOES sheet for removing “small metallic nodules, particles and the like extending through or protruding above the insulative coating”. The '337 patent discloses applying a voltage to a GOES coil disposed in an aqueous sodium nitrate or sodium chloride bath to pit away iron mound defects. Electrolytic processes, however, require additional infrastructure and may significantly increase production costs.
Accordingly, it would be advantageous to provide a novel method for removing or reducing the height of iron mound defects on mill glass coated electrical steel. Alternatively, the method would damage the iron mound defect to the point where it is rough enough to better retain a significant amount of monomagnesium phosphate applied top coating.